1. Field of the Invention
The present invention relates to an internally enhanced heat exchange tube. More particularly, an enhanced flow of heat through the tube wall is achieved by providing the inside of the tube with inwardly projecting, helically disposed, projections separated from adjacent projections by a trough.
2. Description of the Related Art
Large capacity air conditioning and refrigeration (ACR) devices utilize heat exchangers to transfer heat from one fluid to a second fluid. For evaporation cooling, warm water passes over the outside of bundles of heat exchange tubes contained within the heat exchanger while a relatively low vaporization temperature liquid refrigerant such as trichloromonofluoromethane or dichlorodifluoromethane flows through the heat exchange tubes. Heat is extracted from the water causing the refrigerant to evaporate and form vapor. The energy required for evaporation reduces the temperature of the water. External to the heat exchanger, a compressor compresses the vapor and another heat exchanger extracts heat from the vapor, condensing the vapor back to a liquid for return to the first heat exchanger.
The more efficient the transfer of heat from the water outside the heat exchange tubes to the refrigerant inside the heat exchange tubes, the more efficiently and cost effectively the ACR device may be operated.
Some heat exchange tubes have a smooth bore. However, the efficiency of the cooling apparatus is improved when the surface area of the bore is increased. One method for increasing the surface area is to texture the inside wall of the tube.
Such texturing may include projections that extend inwardly from the inner bore of the tube. Known projections include helically disposed fins as disclosed in U.S. Pat. No. 4,658,892 to Shinohara et al. and pyramid-shaped projections as disclosed in U.S. Pat. No. 5,070,937 to Mougin et al. Both the Shinohara et al. patent, including the disclosure of Reexamination Certificate (1256.sup.th) B1 U.S. Pat. No. 4,658,892, and the Mougin et al. patent are incorporated by reference in their entireties herein.
One method of texturing the bore is to draw a smooth walled tube over a textured plug. The plug deforms the internal bore forming a plurality of parallel spiral ridges. The spiral ridges both increase the surface area and create a controlled flow of refrigerant maximizing the liquid phase contact with the tube.
The Shinohara et al. patent discloses that a number of factors influence the transfer of heat through a heat exchange tube. One factor is the height of the projections. The height may be normalized as a ratio of the projection height divided by the inside diameter of the tube.
The Shinohara et al. patent discloses that optimum heat transfer is achieved when the normalized ratio is between 0.02 to 0.03. It also discloses that apex angles less than 30.degree. have poor workability and are not practically manufactured. The same patent suggests a fin height of 0.15-0.20 millimeters.
With a fin height (F.sub.H) limited to 0.15 mm-0.20 mm, the maximum inside diameter (ID) of the tube is limited to about: EQU F.sub.H /ID=0.02 EQU 0.2 mm/ID=0.02 EQU ID=10 mm(0.39 in.)
The limit on the inside diameter of the heat exchange tube is a direct result of the method of manufacture. If an alternative method of manufacture could produce higher fins without tearing or breakage, correspondingly larger inside diameter tubes could be made.
A second factor disclosed by Shinohara et al. is the ratio between the height of a projection and the cross-sectional area of a trough adjacent to the projection. The effective ratio is disclosed as between 0.15 and 0.40 mm. The reference discloses that when this ratio exceeds 0.3 mm, heat transfer abruptly begins to lower.
One alternative method to manufacture internally or externally enhanced heat exchange tubes is disclosed in U.S. Pat. No. 3,906,605 to McLain which is incorporated in its entirety by reference herein. The patent discloses texturing a metallic strip by passing the strip through textured rolls. The strip is then deformed into a generally tubular configuration bringing the edges in close proximity for welding.